As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, research on lithium secondary batteries, which exhibit high energy density and discharge voltage, is underway and such lithium secondary batteries are commercially available and widely used.
In general, secondary batteries have a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator disposed therebetween is accommodated in a laminated or wound form in a battery case made of a metal can or a laminate sheet and an electrolyte is injected thereinto or the electrode assembly is impregnated with an electrolyte.
One of main research subjects in a pouch type secondary battery composed of high-voltage battery cells among such secondary batteries is to prevent performance deterioration of batteries due to leakage of gas when the battery is subjected to activation and in use. For example, during activation, the secondary battery generates a large amount of gas, and the generated gas damages a sealing part of a battery case and is leaked outside. Subsequently, an electrolyte leaks to the damaged sealing part, whereby performance of the battery is deteriorated. In addition, when, during use, the electrolyte is decomposed due to an abnormal battery operation state such as an overcharging exceeding an allowed current and voltage, exposure to high temperature and the like and gas is generated, the battery is damaged as described above.
Therefore, there is an urgent need to develop a novel lithium secondary battery that addresses the problems described above.